Ianus

Proper noun

Janus

1. (Roman) The god of doorways, gates and transitions, and of beginnings and endings, having two faces looking in opposite directions.
   • In the ages of victory, as often as the senate decreed some distant conquest, the consul denounced hostilities, by unbarring, in solemn pomp, the gates of the temple of Janus. Domestic war now rendered the admonition superfluous, and the ceremony was superseded by the establishment of a new...1789, Edward Gibbon, chapter XLI, in The History of the Decline and Fall of the Roman Empire, volume V, London: Printed for A[ndrew] Strahan, and T[homas] Cadell, in the Strand, OCLC 30106274...
2. (attributive) Used to indicate things with two faces (such as animals with diprosopus) or aspects; or made of two different materials; or having a two-way action.
   • Janus cat, Janus particle
   1. (attributive) Used to indicate an azo dye with a quaternary ammonium group, frequently with the diazo component being safranine.
      • Janus green B is a dye used widely in histology to stain cells for microscopic examination.
3. (figuratively) A two-faced person, a hypocrite.
   • The hypocrite; or, The modern Janus.1814, Selina Davenport, The Hypocrite; or, The Modern Janus. A Novel. In Five Volumes, volume I, London: Printed at the Minerva-Press, for A. K. Newman and Co. Leadenhall-Street, →OCLC, title page
4. A moon of Saturn.
   • Heavily cratered and irregularly shaped, Janus orbits Saturn just beyond the F ring and only 50km (30 miles) farther away than its co-orbital moon, Epimetheus.2012 October, “Saturn”, in Martin Rees, editor, Universe, rev. edition, London: Dorling Kindersley, →ISBN, page 192
5. (rare) A male given name from Latin.